1. Field of the Invention
The invention relates to the establishing of multimedia sessions. Particularly, the invention relates to a method for the establishing of peer-to-peer multimedia sessions in a communication system.
2. Description of the Related Art
Due to the lack of a sufficient number of Internet Protocol (IP) version 4 addresses several methods for address translation have been introduced. The common factor in these methods there is a logical separation of a physical network continuum to a first network and a second network, which are connected with a node that allocates addresses from the second network on demand for establishing communications between first and the second networks. The first network is assumed to be a network which may use IP addresses that overlap with the IP addresses in other networks, whereas the second network is a large network where the overlapping use of addresses is not an option due to the connectivity to other networks. The first network is usually a corporate network. The second network is usually a backbone network. One such method is referred to as the Network Address Translation (NAT). NAT is used to enable multiple hosts on the first network to access the Internet or generally the second network using a single IP address from the second network.
The problem associated with NAT is that nodes behind a NAT-enabled router, that is, the address allocating node, do not have true peer-to-peer connectivity with a remote party and cannot use some Internet protocols such as the Session Initiation Protocol (SIP) in a peer-to-peer fashion. Services that require the initiation of the Transmission Control Protocol (TCP) connections from the outside network or stateless protocols such as those using the User Datagram Protocol (UDP) may become disrupted due to the use of NAT. Unless a NAT-enabled router makes a specific effort to support these protocols, incoming packets cannot reach their destinations behind the NAT-enabled router. Sometimes application layer gateways are used to provide connectivity over a NAT-enabled router. In other words, the internal addresses for nodes in the first network are not visible for nodes in the second network.
In a pure peer-to-peer Session Initiation Protocol (SIP) environment a further problem is that the SIP messages may contain IP addresses and port numbers and a node in a first network has no way of knowing the addresses to be obtained from the second network. A benefit of being able to communicate in an end-to-end fashion would be to avoid large investments in Session Initiation Protocol servers and media stream proxies.